How to Use a Star Point for Mixed Signal Grounding
The Mixed Signal Grounding Problem
Grounding mixed signal PCBs is a complex issue with many problems and solutions. You need to keep your analog and digital signals separate to reduce EMI, but they also need to be grounded together. Improper PCB mixed signal grounding can result in large ground loops that will radiate noise in, and possibly around, your circuit.
The main ground plane problem on mixed signal boards is digital circuits. Digital switching chips can be quite noisy, which would not be a problem if they were alone. However, when paired with analog circuits, digital EMI will often find itself mingling with sensitive analog signals. That’s why you should generally try to keep these two systems separated.
Separating analog and digital circuits creates another issue--a floating ground. Your analog and digital chips all need to be referenced to the same ground plane in order to operate properly. If you connect your separate analog and digital ground planes wherever you want, you can create ground loops. Large ground plane loops will act like antennae and radiate EMI to other of your board, and possibly out of your device. A star grounding for PCB mixed signal grounding allows you bring both your analog and digital circuits together at a single point. This reduces the possibility of ground loops and EMI radiation.
All ground connections should terminate at your star ground.
A star ground is a single point that connects analog and digital grounding planes to eliminate ground loops. It doesn’t necessarily have to be in the shape of a star, but the idea is that each connection is made at a single central point.
Your analog and digital circuits should interact as little as possible. You should use separate power supply rails for each, and connect them only at the star point. This will ensure that ground loop areas are minimized and that analog and digital signals don’t mix and interfere with each other in the ground planes.
When to Use Star Grounds
The perfect time to use a star ground is when you have a single PCB with a single ADC/DAC on it. In that kind of system, you can make your star point the grounding point for your analog-to-digital converter. Then, connect your analog ground plane and digital ground plane on either side of the converter at that same point as well. This will help you greatly reduce noise in your mixed signal circuit and make grounding a breeze.
If you’re designing a multi-card system, or are using more than one converter, a star ground might not work for you. The problem is that you’ll have more than one star point, creating the potential for lots of ground loops. The whole idea of a star ground is that everything is connected at one point. If you can’t connect everything at one point, don’t even try.
Keep your circuit well grounded.
If you don’t properly ground your mixed signal circuit to the ground plane correctly, things will get noisy. A star ground can help reduce EMI on your PCB and keep everything in its right place. However, you should take care to only use a star ground in systems with a single converter. Once you get into more complex systems creating a single star point becomes almost impossible.
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